1. Field of the Invention
The present invention relates to the field of methods of cleaning a deposition chamber, and more specifically, to a method of removing silicon deposition by-products from internal chamber components and chamber exhaust lines while minimizing the effect on quartz chamber components.
2. Discussion of Related Art
In epitaxial or polysilicon deposition processes, silicon is typically deposited onto the wafer using processes such as chemical vapor deposition (CVD). A cross-sectional view of a typical single wafer, cold-wall CVD apparatus is shown in FIG. 1. The Figure shows a thermal reactor 100 for processing semiconductor substrates comprising, a double-dome reactor vessel 114 principally made of quartz that defines a reactor chamber 102, upper 118 and lower 104 quartz chamber liners, gas inlet manifold 106, a gas exhaust manifold 108, exhaust line 109, a radiant heating system 128, a drive assembly 126, a susceptor 124, a wafer 125, and a preheat ring 130. Susceptor 124 is larger than wafers processed. For example, a 200 mm wafer would be processed on a 240 mm susceptor. Additionally, but not shown, is that the gas inlet 106 is coupled to a gas supply of silicon, for example, dichlorosilane (DCS) or monosilane (SiH.sub.4), or silane tetrachloride (SiCl.sub.4), or trichlorosilane (TCS) and chlorine. Not shown in FIG. 1 is the pumping means coupled to exhaust line 109 for exhausting chamber. One of ordinary skill in the art will appreciate that chamber processes could be practiced at atmosphere pressure which would not require pumping means to provide chamber exhaust. One of ordinary skill will also appreciate that chamber processes could be performed at reduced pressures by utilizing a pumping means to lower chamber pressure. The illustrated reactor 100 does not use a plasma for either deposition or cleaning. The double dome rector vessel 114 includes a top dome 120 and a bottom dome 116, of quartz which are cooled by circulating cooling air around the outer surface of the quartz. Additionally, cooling water is circulated through the walls of the reactor such that a cold-wall, i.e., T.sub.wall &lt;T.sub.process, is maintained. Typical dome temperatures range from about 100.degree. C.-600.degree. C. The drive assembly 126 is coupled to a motor (not shown) to rotate the susceptor 124 during the deposition process to enhance coating uniformity of the wafer 125 supported on top of the susceptor 124. Temperature measured at the susceptor is commonly used and referred to as process temperature, chamber temperature, deposition temperature or susceptor temperature.
The cold-wall is an important feature of the single wafer system since it reduces the deposition of semiconductor materials on the interior surface of the upper 120 and lower 116 quartz domes.
After repeated deposition processes are conducted in chamber 102, top surfaces of the circumferential edge of the susceptor 124 and the preheat ring 130 are covered with a film of the deposited material. The susceptor 124 is usually constructed of a thin plate for low thermal mass and a surrounding rim for rigidity. The diameter of a susceptor in a typical reaction vessel is approximately 1.5 inches larger than the diameter of the wafers being processed. A typical susceptor diameter for 200 mm wafers, for example, would be about 240 mm. Even though other wafer diameters are processed such as 150 mm or 300 mm, the susceptor diameter is always larger than the wafer diameter. Thus, a circumferential area is therefore exposed to the depositing semiconductor material. The susceptor 124 is typically made of graphite and coated with a silicon carbide coating such that it can be heated up to the deposition temperature without significant contamination.
The preheat ring 130 substantially seals the gap between the susceptor 124 and the quartz liner 104 and 118 of the reactor vessel 102 to control the heat lost from the edge of the susceptor. The preheat ring 130 is made of graphite material coated with silicon carbide for absorbing energy from radiant heating system 128. A quartz preheat ring can also be used. The top surface 129 of preheat ring 130 is exposed to the deposition material and therefore accumulates a film of such material due to the fact that the ring is heated to the deposition temperature.
In this process, a reactant gas mixture including a silicon source (such as silane, disilane, dichlorosilane, trichlorosilane, or silicon tetrachloride) is heated and passed over the wafer to deposit silicon film on the wafer surface. In most instances, a carrier gas, such as hydrogen, is also injected into the processing chamber. It is well known that after a sufficient number of deposition processes, i.e., about 1-10 processes for most materials, a film of sufficient thickness in the range between 1-10 microns builds up on the susceptor and preheat ring. In some chambers H.sub.2 or other gas provides positive pressure to the backside of the susceptor and preheat ring to prevent backside silicon deposition. If backside pressure or some other method of preventing backside deposition is not employed, unwanted deposits will form on the backside of the susceptor and preheat ring as well. These unwanted films, if not removed, can impede the heating efficiency of the graphite parts. Most chambers of the CVD cold-wall type consist of quartz domes that form the upper and lower chamber boundaries. The quartz walls are kept cool relative to the susceptor process temperature by providing cooling air across the outer dome surface. Thus, silicon formation on the quartz is reduced. However, deposits will form on the quartz domes which could interfere with radiant energy transmission from radiant lamp system 128. Just like the deposits on other chamber components, these deposits could also flake or peel over time becoming sources of contamination and disrupt the process integrity. After several deposition process cycles the accumulated, unwanted deposition must be removed from the chamber. The accumulated material silicon and silicon containing polymer- is essentially the same regardless of whether the deposition result is epitaxial, amorphous or polysilicon film.
These silicon accumulations are typically removed with hydrogen chloride (HCl). In this process, the chamber is heated from the wafer transfer temperature to a temperature of about 1200.degree. C. Once the chamber reaches 1200.degree. C., above the dissociation temperature of HCl gas, HCl is introduced into the chamber. As a result of the high temperature, the HCl dissociates into reactive hydrogen (H) and chlorine (Cl) which will react with the silicon byproducts. The Cl radicals also react with quartz process parts like the quartz dome and liner. Since deposition rate is proportional to temperature of the surface and the cooling air keeps the dome several hundred degrees cooler than the susceptor, the quartz dome accumulates deposits at a slower rate than they accumulate on the susceptor and preheat ring. Thus, the quartz components are usually coated to a much lesser degree than other chamber components and could be susceptible to damage from the chlorine radicals with energy levels beyond thermal reaction energy. Once the silicon accumulations have been removed, the HCl flow is stopped and the chamber once again cooled from clean temperature of about 1200.degree. C. to a wafer transfer temperature. Although HCl removes silicon deposits from the chamber components, it does not remove them completely from the system. Instead, when the H/Cl/Si mixture reaches the relatively cooler exhaust line it condenses forming a chlorosilane polymer. This polymer accumulates in the exhaust line after each clean. Furthermore, below it's dissociation temperature, (1150.degree. C.) HCl will not break up the polymer. Thus, any HCl that reaches the exhaust line will not remove the polymer but would likely contribute to polymer formation. In time, the exhaust line polymer build up is so significant that the chamber must be taken out of production and the exhaust line cleared.
Thus, what is desired is a method of removing silicon deposits from a cold-wall CVD silicon deposition reactor with a process that does not utilize high temperatures and the necessary thermal transients; can remove chamber deposits without harm to internal chamber components; and can react with and remove polymer residue in the chamber exhaust line.